The establishment of positional identity along the mediolateral and anteroposterior axes is a crucial step in the development of the vertebrate nervous system. Recently several genes have been characterized which display regionalized expression along the mediolateral axis of the neuroectoderm in Xenopus embryos. These genes include XFKHI/pintallavis, which is expressed at the axial midline, XASH3, which is expressed at the midpoint of the mediolateral axis, and Xsnail, which is expressed at the lateral edge of the neuroectoderm. The restricted expression of these genes indicates that regionalization of the mediolateral axis is initiated at gastrula stages of development, well before the onset of neurulation. The design of this grant is to use these genes, as well as others to be isolated in the initial phase of this project, as markers in studies aimed at determining the signals responsible for mediolateral pattern formation within the neuroectoderm. The contributions of signals from the mesoderm and ectoderm to patterning in intermediate and lateral regions of the neuroectoderm will be assayed using two approaches. First, marker gene expression will be examined in manipulated embryos in which these potential signalling sources are absent. In addition, tissue recombinations between the presumptive neuroectoderm and the ectoderm and mesoderm will establish the ability of each to generate mediolateral pattern. The definition of the specific signalling source(s) in these assays will allow the initiation of studies aimed at characterizing these signals in molecular detail. Novel homologs of the wnt, hedgeghog and TGFbeta families of signalling molecules will be isolated from the identified signalling source(s) and assayed for mediolateral patterning activity. In this manner, both the specific tissue interactions and molecular signals required for patterning in intermediate and lateral regions of the neuroectoderm will be elucidated.